1. Field of the Invention
The present invention relates to a transition metal composite hydroxide capable of serving as a precursor of a positive electrode active material for nonaqueous electrolyte secondary batteries, a method for producing the same, the positive electrode active material for nonaqueous electrolyte secondary batteries, a method for producing the positive electrode active material for nonaqueous electrolyte secondary batteries, and a nonaqueous electrolyte secondary battery using said positive electrode active material.
2. Description of the Related Art
In recent years, with the spread of portable electronic equipment, such as cell phones and notebook-sized personal computers, development of a small and lightweight nonaqueous electrolyte secondary battery having a high energy density has been strongly desired. Also, development of a high-output secondary battery as a battery of a power source for driving motors, particularly a power source for transport equipment, has been strongly desired.
As such a secondary battery, there is a lithium-ion secondary battery. The lithium-ion secondary battery comprises a negative electrode, a positive electrode, an electrolyte solution, and the like, and, as active materials of the negative electrode and the positive electrode, materials capable of desorption and insertion of lithium are used. At present, research and development of such lithium-ion secondary battery are being actively conducted, and particularly, since a 4V class high voltage can be achieved by a lithium-ion secondary battery using a lithium metal composite oxide having a layered or spinel structure as a positive electrode material, the commercialization thereof as a battery having a high energy density is progressing.
As a material which has been mainly proposed until now, it may include lithium-cobalt composite oxide (LiCoO2), which is relatively easily synthesized; lithium-nickel composite oxide (LiNiO2), in which nickel, more inexpensive than cobalt, is used; lithium-nickel-cobalt-manganese composite oxide (LiNi1/3Co1/3Mn1/3O2); lithium-manganese composite oxide (LiMn2O4), in which manganese is used; and the like. Among these, lithium-nickel-cobalt-manganese composite oxide has been highlighted as a material having excellent cycle characteristics and providing high output with low resistance.
In order to achieve such good performance, it is required to use a lithium composite oxide having a uniform particle diameter as a positive electrode active material.
The reason for this is that use of a composite oxide having a wide particle size distribution causes unevenness of voltage applied to particles in an electrode, whereby fine particles selectively degrade when charge and discharge are repeated, and cycle deterioration is easily caused. Therefore, in order to improve the performance of a positive electrode material, it is important to produce a lithium composite oxide having a small and uniform particle diameter as a positive electrode active material.
In other words, a lithium composite oxide is usually produced from a composite hydroxide, and therefore, in order to improve the performance of a positive electrode material and produce a lithium-ion secondary battery having high performance as a final product, a composite hydroxide including particles having a small particle diameter and a narrow particle size distribution needs to be used as a composite hydroxide to be used as a raw material of a lithium composite oxide constituting a positive electrode material.
As for a particle size distribution of a lithium composite oxide, for example, Japanese Patent Application Laid-Open No. 2008-147068 discloses a lithium composite oxide, the lithium composite oxide being particles having a particle size distribution wherein, in a particle size distribution curve, an average particle diameter D50, which represents a particle diameter having a cumulative frequency of 50%, is 3 to 15 μm, a minimum particle diameter is not less than 0.5 μm, and a maximum particle diameter is not more than 50 μm, and furthermore, in a relationship between D50 and D10 as well as D50 and D90, the D10 having a cumulative frequency of 10% and the D90 having a cumulative frequency of 90%, D10/D50 and D10/D90 are 0.60 to 0.90 and 0.30 to 0.70, respectively. It is also disclosed that this lithium composite oxide has a high density, is excellent in charge-and-discharge capacity characteristics and output characteristics, and is resistant to degradation even under heavy charge-and-discharge load conditions, and therefore use of the lithium composite oxide allows a lithium ion nonaqueous electrolyte secondary battery having excellent output characteristics and less degradation of cycle characteristics to be obtained.
However, the lithium composite oxide disclosed in Japanese Patent Application Laid-Open No. 2008-147068 has an average particle diameter of 3 to 15 μm while having a minimum particle diameter of not less than 0.5 μm and a maximum particle diameter of not more than 50 μm, and hence includes very fine particles and coarse particles.
Moreover, it cannot be said that such particle size distribution specified by the above-mentioned D10/D50 and D10/D90 is a narrow particle size distribution.
In other words, it cannot be said that the lithium composite oxide disclosed in Japanese Patent Application Laid-Open No. 2008-147068 is particles having sufficiently high uniformity of particle diameters, and hence, even if such lithium composite oxide is adopted, improvement in performance of a positive electrode material cannot be expected, and it is difficult to obtain a lithium ion nonaqueous electrolyte secondary battery having sufficiently high performance.
Also, there has been proposed a method of producing a composite hydroxide to be used as a raw material of composite oxide, in order to improve a particle size distribution. Japanese Patent Application Laid-Open No. 2003-86182 proposes a method of producing a positive electrode active material for nonaqueous electrolyte batteries, wherein an alkaline solution is introduced together with an aqueous solution containing two or more of transition metal salts or two kinds or more of aqueous solutions of different transition metal salts into a reaction vessel to obtain a hydroxide or an oxide as a precursor through coprecipitation with a reductant being coexistent or an inert gas being supplied.
However, the technique disclosed in Japanese Patent Application Laid-Open No. 2003-86182 aims to classify and collect formed crystals, and therefore, in order to obtain a product having a uniform particle diameter, production conditions need to be strictly controlled and it is hard to implement an industrial scale production. In addition, even if crystal particles having a large particle diameter can be obtained, it is difficult to obtain particles having a small particle diameter.
Further, in recent years, there have been made efforts to further improve the performance by adding various elements. Tungsten acts to reduce reaction resistance, whereby the effect of achieving high-output can be expected. For example, Japanese Patent Application Laid-Open No. H11-16566 proposes a positive electrode active material which is coated with a metal containing at least one element selected from the group consisting of Ti, Al, Sn, Bi, Cu, Si, Ga, W, Zr, B, and Mo and/or an intermetallic compound obtained by a combination of a plurality of the above mentioned elements, and/or an oxide. Japanese Patent Application Laid-Open No. H11-16566 describes that such coating enables oxygen gas to be absorbed and safety to be secured, but does not disclose output characteristics at all. Moreover, the disclosed manufacturing method is coating using a planetary ball mill, and such coating method gives physical damages to the positive electrode active material, and causes decrease in battery characteristics.
Japanese Patent Application Laid-Open No. 2005-251716 proposes a positive electrode active material for nonaqueous electrolyte secondary batteries, the positive electrode active material having at least a lithium transition metal composite oxide having a layered structure, wherein the lithium transition metal composite oxide exists in a particle form comprising either or both of primary particles and secondary particles composed of aggregation of the primary particles, and has a compound comprising at least one element selected from the group consisting of molybdenum, vanadium, tungsten, boron, and fluorine, at least on the surface of said particles.
Thus, Japanese Patent Application Laid-Open No. 2005-251716 provides a positive electrode active material for nonaqueous electrolyte secondary batteries, the positive electrode active material demonstrating excellent battery characteristics even under severer environment conditions for use, and particularly, describes the improvement of initial characteristics without loss of improvements in thermal stability, load characteristics, and output characteristics since the positive electrode active material has, on the surface of the particles, a compound comprising at least one selected from the group consisting of molybdenum, vanadium, tungsten, boron, and fluorine.
However, Japanese Patent Application Laid-Open No. 2005-251716 describes that the effect of at least one addition element selected from the group consisting of molybdenum, vanadium, tungsten, boron, and fluorine exists in improvement in initial characteristics, that is, initial discharge capacity and initial efficiency, and thus it does not refer to output characteristics. Also according to the disclosed manufacturing method, the addition element is mixed and burned with a hydroxide which has been heat-treated simultaneously together with a lithium compound, and therefore there is a problem that a part of the addition element substitutes for nickel which has been arranged in layers, whereby battery characteristics decrease.
In view of such problems, the present invention aims to provide a transition metal composite hydroxide, the use of which as a precursor allows a lithium transition metal composite oxide having a small particle diameter and high uniformity of particle diameters to be obtained.
Also, the present invention aims to provide a positive electrode active material for nonaqueous secondary batteries, the positive electrode active material being capable of reducing positive electrode resistance values measured when used for batteries, and also to provide a nonaqueous electrolyte secondary battery including said positive electrode active material, the secondary battery having high capacity and thermal safety, and achieving high output.
Furthermore, the present invention aims to provide a method for industrially producing the transition metal composite hydroxide and the positive electrode active material according to the present invention.